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LDAP Injection Prevention

LDAP injection stems from concatenating untrusted input into search filters. Prevent it with proper escaping, strict validation, and safer auth flows.

Escaping Special Characters

Escape these characters in filter values (RFC 4515):

  • ( → \28
  • ) → \29
  • * → \2a
  • \ → \5c
  • NUL → \00

Note: DN components require DN-safe escaping (RFC 4514); use library helpers with DN mode.

PHP Example (filter-based auth)

Vulnerable (no escaping):

$filter = '(&(cn=' . $_POST['username'] . ')(userPassword=' . $_POST['password'] . '))';

Safe (escape user input):

$filter = '(&(cn=' . ldap_escape($_POST['username']) . ')(userPassword=' . ldap_escape($_POST['password']) . '))';

Prefer Bind-Based Authentication

Delegate password checking to the DS by binding with user-supplied credentials instead of searching with a filter:

$dn = 'cn=' . ldap_escape($_POST['username'], '', LDAP_ESCAPE_DN) . ',dc=example,dc=htb';
$pw = $_POST['password'];
$bind = ldap_bind($conn, $dn, $pw);

This removes the need to compose a filter for password checks, reducing injection risk.

Best Practices

  • Least privilege: bind account for searches should have minimal read rights.
  • Input allow-listing: restrict to expected character sets and formats.
  • Fixed enums: map selector-like inputs to constant strings (no free-form joining).
  • Centralize validation/escaping: avoid scattered ad-hoc handling.
  • Disable anonymous binds on the DS.
  • Log rejects/rate-limit abnormal patterns.

References

  • RFC 4515 (LDAP: String Representation of Search Filters)
  • RFC 4514 (LDAP: String Representation of Distinguished Names)
  • PHP ldap_escape documentation